The need to protect electronic circuitry from overvoltages, especially transient overvoltage conditions, is well known. Most electronic components are built to withstand the application of certain limited voltages thereacross, and thus, may be permanently damaged or at least seriously malfunction if far higher voltages are applied.
There are many sources of transient overvoltages, such as lightning, inductive surge, electrostatic discharge (ESD), and electromagnetic induction (EMI). Further, failure of one "upstream" circuit component of a system may allow excess voltages to be applied across other "downstream" circuit components, resulting in a "domino effect" breakdown of circuit components within that system.
Lightning, ESD and inductive surges are all capable of producing very rapid high voltage transients. An inductive surge produced, for example, by interrupting a running 115 volt motor can be as high as 1,000 volts or more. Electrostatic discharges, such as those produced by a person walking on a wool rug on a dry winter day, can easily result in charging to tens of thousands of volts. Although such electrostatic discharges usually involve a relatively small total energy, they, like inductive surges, are sufficient to destroy many types of microelectronic circuits. Overvoltage transients caused by lightning can deliver, by direct strikes, large currents at tens of thousands to hundreds of thousands of volts. Additionally, by the process of EMI, lightning can generate high voltage, broad spectrum transients with components in the megahertz frequency range and higher.
Conventional devices employed for dealing with relatively small overvoltages include shunting capacitors, breakdown diodes, varistors and inductive coils, all of which devices include various types of switching means and materials. Breakdown diodes such as zener diodes, when reverse biased beyond a certain threshold voltage, conduct large currents. Like virtually all overvoltage protection devices, such a diode is operatively disposed "upstream" of or in parallel with a circuit element to be protected, and is thereby effective in shunting excess voltage applied thereacross to a discharge path such as a neutral line, D.C. common line, chassis or ground. However, such diodes are only capable of handling limited overvoltages without becoming permanently damaged themselves.
Varistors, which are typically made of pressed powders, act somewhat like zener diodes in that they offer a high impedance at low voltages and a relatively low impedance at high voltages. However, varistors are distinguishable from zener diodes in that the current characteristics thereof are symmetrical rather than asymmetrical, and thus can offer limited protection against overvoltage in both directions.
Inductive coils or chokes, while unable to protect circuitry from low frequency or static overvoltages, do tend to filter out rapid voltage transients by presenting a large impedance. Since they also present high impedance to high frequency signals, they are inappropriate for protecting high frequency circuitry from high frequency overvoltages. However, such devices tend to be relatively bulky and expensive.
Spark gaps are another form of overvoltage protection associated with higher power devices. Recently, miniaturized forms of such spark gaps have been developed for use on P.C. boards and the like. Spark gaps contain two opposing electrodes separated by a gas, such as air, which has a desired breakdown, or sparking voltage. When an overvoltage is applied across the spark gap, its nonconductive gas becomes ionized, forming a relatively low resistance path between the electrodes thereof. Although spark gaps have beneficial uses, they usually are not appropriate for use in solid-state circuitry because they are not solid-state devices and because they are fairly large, even in miniaturized form. Also the time required for the operation of spark gaps is usually too slow to provide full protection from extremely rapid transients.
Varistors, inductive coils and spark gaps all share the same shortcoming, i.e., they cannot be readily incorporated into microelectronic devices due to the required manner in which they must be made. Thus, there remains a need for extremely high speed and/or high power surge protection of circuit elements that can be incorporated directly into all types of microelectronic circuitry, as an integral part thereof, to protect such circuitry from rapid, overvoltage transient surges.
In addition to the more common forms of transient surges mentioned hereinabove, the nuclear age has created a new source of overvoltage transients known as nuclear electromagnetic pulse (hereinafter EMP), which pulses pose a serious threat to national security. EMP is produced by Compton electrons scattered by gamma rays from a nuclear explosion colliding with air molecules of the upper atmosphere. Theoretical studies have indicated that if a nuclear device were exploded at a high altitude above most of the earth's atmosphere a large EMP generated therefrom would have sufficient intensity to induce a large current in conductors hundreds or thousands of miles away and thereby destroy electronic equipment connected to or containing such conductors.
EMP is particularly difficult to protect against for three reasons: (1) the extremely rapid rise time; (2) the expected intensity, and (3) the ubiquitous presence, i.e., all conductors of any appreciable length not enclosed with a suitable Faraday shield will act as an antenna, and thus be subject to severe electrical transients. It has been estimated that EMP will produce an extremely high overvoltage within approximately one nanosecond or less and reach a peak field in about 10 nanoseconds, before trailing off in about one microsecond. The peak field produced by a one-megaton warhead exploding in the upper atmosphere may be as high as 50,000 volts/meter. Further details about the nature of EMP and the inadequacies of conventional overvoltage protection devices to guard against them is found in "Electromagnetic pulses: potential crippler," IEEE Spectrum, May, 1981, pp. 41-46.
Most conventional solid-state overvoltage protection devices are too slow or limited in their power handling capabilities to provide full protection against the effects of very close lightning strikes or EMP. This is because such lightning strikes and EMP can produce overvoltages two or three orders of magnitude or more above the normal operating voltages of the integrated circuits subjected to such transients, thus leading to enormous current surges capable of destroying virtually all types of solid-state semiconductor protection devices. As the energy content of such pulses is increased, the problem becomes more severe, and requires extremely rugged, high capacity overvoltage protection devices, preferably incorporated at the integrated circuit level, to handle any transients which reach such microelectronic circuits. As the size of microelectronic circuit elements is reduced, the problem also becomes more severe since less energy is required to damage smaller devices. As an additional restriction, it is necessary that overvoltage protection devices, when inserted into or included as part of the electronic circuit to be protected, must not impose undue insertion losses in the circuit, or decrease switching speeds or band width by adding significant amounts of capacitance.
One class of overvoltage protection materials which has held great potential for very high speed transient suppression applications are Ovonic threshold switching materials of the type first invented by S. R. Ovshinsky in the 1960's. U.S. Pat. Nos. 3,171,591 (1966) and 3,343,034 (1967) specifically teach that these types of threshold switching materials, as well as the devices incorporating said materials, are suitable for use as surge suppressors, such as for transient inductive pulses and the like. Said materials have been known since at least 1968 to have a switching speed of less than 150 picoseconds, see, e.g., S. R. Ovshinsky, "Reversible Electrical Switching Phenomena in Disordered Structures", Physical Review Letters, Vol. 21, No. 20, Nov. 11, 1968, p. 1450(c).
Ovonic threshold switching materials, as generally described for the purposes enumerated herein, exhibit a bistable characteristic, including a threshold voltage and a minimum holding current. Specifically, the devices are constructed to include a semiconductor material with at least a pair of electrodes operatively disposed on opposite sides thereof. The semiconductor material is designed to have a threshold voltage value and a high electrical resistance so as to provide a high resistance state adapted to substantially block current flow therethrough. The high electrical resistance state is initially encountered in response to a voltage above the threshold voltage value and very rapidly decreases (in at least one path between the electrodes) to a low electrical resistance, which is orders of magnitude lower than the high electrical resistance, thus providing a low resistance path for conducting current through the semiconductor material. The current conducting condition is maintained so long as at least a minimum holding current continues to pass through the conducting path through the material. When the current falls below this minimum current value, the material rapidly reverts to its high resistance blocking condition. The voltage drop across the semiconductor material in a threshold switch, when in its conducting condition, is a fraction of the voltage drop across the material when in its high electrical resistance blocking condition, as measured near the threshold voltage value of the switching material.
Many different combinations of atomic elements, when combined in the proper proportions and manner, have been shown to produce a semiconductor material exhibiting the aforementioned threshold switching characteristics. Most commonly, chalcogenide glass semiconductor materials, such as Te.sub.36 Ge.sub.23 S.sub.21 As.sub.18 Se.sub.2 and Te.sub.39 As.sub.36 Si.sub.17 Ge.sub.7 P, may be used. Examples of other such materials and threshold switching devices made therefrom can be found in the following list of U.S. patents, all of which are assigned to the assignee of the present invention, and all of which are hereby incorporated by reference:
______________________________________ 3,271,591 3,571,671 3,343,034 3,571,672 3,571,669 3,588,638 3,571,670 3,611,063 ______________________________________
While the above mentioned materials, when utilized to fabricate a threshold switch, possess favorable threshold switching characteristics, several limitations have prevented them from gaining wide acceptance for use as surge suppressors. In the first place, it is required that the chalcogenide materials from which said surge suppressing switches are fabricated exhibit a high degree of stability and uniformity so that threshold voltages and holding voltages can be reliably and uniformly set. It is necessary that the switching stability of the chalcogenide glass material remain constant despite the passage of time or through repeated use. Also, when utilized in a large matrix array, it is essential that the switching characteristics of discrete switches remain uniform from switch to switch in that matrix. Heretofore, uniformity of switching was not within acceptable tolerances, with the variance in switching characteristics being the most significant in the observed values of the difference between the first fire voltage and the threshold voltage. As used herein, the threshold voltage is defined as that voltage at which the thin film chalcogenide glass material switches from its high resistance state to provide at least one low resistance path for conducting excess current therethrough. Also, as used herein, first fire voltage is defined as the threshold voltage observed the first time the thin film chalcogenide glass material is exposed to an overvoltage.
From the foregoing discussion, it should be apparent that having uniform, reproducible values for these two switching characteristics (first fire voltage and threshold voltage) is essential to developing and manufacturing reliable surge protection devices. It is to the ultimate development of modified, thin film, chalcogenide glass switching material, which material exhibits uniform, stable threshold and first fire voltage characteristics that the instant invention is directed.